1. Field of the Invention
The present invention relates to a communication device and more particularly to a communication device using a communication method of simultaneous transmission and reception of a plurality of carriers.
2. Related Art
Recently, multiplexing carrier methods are used for improving the wireless communication speed. In particular, OFDM (orthogonal frequency division multiplexing) method uses a plurality of sub-carriers (carriers) placed on a frequency axis, the interval between adjoining two sub-carriers among the plurality of sub-carriers is implemented as the theoretical minimum. The OFDM method has thereby become one good candidate of orthogonal multiplexing carrier methods that allows the usage rate of frequencies to be improved.
The data structure of OFDM signals of orthogonal multiplexing carrier method includes a guard interval at the beginning. The period of time of the guard interval (the length of the guard interval) is preset so as to be longer than the delay at the reception of OFDM signals by taking into account the wireless communication environment (for example, indoor wireless communication environment for placing a transmitter and a receiver in a room). This enables demodulation of OFDM signals when the reception has a delay, if the delay is shorter in comparison with the guard interval time.
Communication devices using such an orthogonal multiplexing carrier method are devised so as to use in an indoor wireless communication environment between the transmitter and the receiver. However, in an outdoor environment the radio wave (OFDM signals) between a transmitter and a receiver is reflected by a variety of obstacles (for example, a building), and is carried from the transmitter to the receiver through a plurality of propagation paths. The path lengths of these propagation paths differ so that the receiver receives a plurality of radio waves with various delays.
That is, a plurality of radio waves to be received (for example the direct radiation wave, delayed wave, and so on) is received by the receiver with different delays. More specifically, in an outdoor environment, the probability of estimated time of arrival of delayed waves beyond the guard interval is higher than in an indoor environment. The receiver in turn has a problem that it cannot recognize the receiving radio waves delayed far more than the guard interval time correctly as the receiving radio waves.